Since its discovery in the fundus (rumen) of the rat stomach, the 5-HT2B receptor has been assumed to be expressed in the gut only by smooth muscle. The PI has recently observed, however, that mRNA encoding the 5-HT2B receptor is also expressed in the intestine; moreover, she has cloned the 5-HT2B receptor from isolated ganglia of the guinea pig myenteric plexus, as well as from the small intestine of the rat and mouse. MRNA encoding the receptor is selectively missing from the aganglionic terminal colon of ls/ls mice, suggesting that intestinal 5-HT2B expression, in contrast to that of the stomach, may be in neurons. Although such cells are relatively rare in adults, they are abundant in fetal mice. 5-HT2B receptors appear in enteric ganglia at E15 and are abundant in virtuall all myenteric ganglia at E16. At this time, ganglia contain neural precursors that coexist with mature serotonergic neurons (among the first enteric neurons to be born). E15-16 is also when 5-HT-secreting EC cells appear in the mucosa. The timing of 5-HT2B expression, its developmental regulation in enteric ganglia, and its ability to induce proliferation when expressed by transfected cells, suggested that stimulation of the 5-HT2B receptor might enable 5-HT to influence the enteric neuronal development. This hypothesis was strongly supported by the observation that 5-HT (1-10 uM) and the 5-HT2 agonist DOI (1 uM), promote the development of neurons in vitro, when added to dissociated cells from the E13-16 fetal mouse intestine. The effect of 5-HT and DOI was abolished by methysergide and ritanserin, which are antagonists at 5-HT2B receptors, but not by ketanserin, a 5-HT2A antagonist. The PI now proposes to test the hypothesis that 5-HT, released either by the early-developing serotonergic neurons or EC cells, exerts a growth factor-like effect through 5-HT2B receptors that is physiologically important in the development of the enteric nervous system (ENS). She will test this hypothesis by: (i) identifyin the cells in developing gut that express the 5-HT2B receptor, (whether they ar of neural crest origin and , if so their lineage); (ii) identifying the cells upon which 5-HT acts to promote enteric neuronal development and whether that effect is due to enhanced proliferation, differentiation and/or survival; (iii determining whether early expression and developmental regulation, like that o enteric 5-HT2B receptors are also exhibited in the gut by other 5-HT receptor subtypes; (iv) identifying the properties (if any) of the ENS that are abnorma in "knockout mice" lacking the 5-HT2B receptor.